The forms ofpseudopodia, from left: polypodial and lobose; monopodial and lobose; filose; conical; reticulose; tapering actinopods; non-tapering actinopods
Amoebae do not have cell walls, which allows for free movement. Amoebae move and feed by using pseudopods, which are bulges ofcytoplasm formed by the coordinated action ofactinmicrofilaments pushing out theplasma membrane that surrounds the cell.[13] The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another.Amoebozoan species, such as those in the genusAmoeba, typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section.Cercozoan amoeboids, such asEuglypha andGromia, have slender, thread-like (filose) pseudopods.Foraminifera emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as theRadiolaria andHeliozoa, have stiff, needle-like, radiatingaxopodia (actinopoda) supported from within by bundles ofmicrotubules.[3][14]
"Naked" amoeba of the genusMayorella (left) and shell of thetestate amoebaCylindrifflugia acuminata (right)
Free-living amoebae may be "testate" (enclosed within a hard shell), or "naked" (also known asgymnamoebae, lacking any hard covering). The shells of testate amoebae may be composed of various substances, includingcalcium,silica,chitin, or agglutinations of found materials like small grains of sand and thefrustules ofdiatoms.[15]
To regulateosmotic pressure, most freshwater amoebae have a contractilevacuole which expels excess water from the cell.[16] Thisorganelle is necessary because freshwater has a lower concentration ofsolutes (such as salt) than the amoeba's own internal fluids (cytosol). Because the surrounding water ishypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane byosmosis. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst. Marine amoebae do not usually possess a contractile vacuole because the concentration of solutes within the cell are in balance with thetonicity of the surrounding water.[17]
The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and otherprotists. Some aredetritivores and eat dead organic material.
Amoebae typically ingest their food byphagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth orcytostome, and there is no fixed place on the cell at which phagocytosis normally occurs.[18]
Some amoebae also feed bypinocytosis, imbibing dissolved nutrients throughvesicles formed within the cell membrane.[19]
Foraminifera have reticulose (net-like) pseudopods, and many species are visible with the naked eye.
The size of amoeboid cells and species is extremely variable. The marine amoeboidMassisteria voersi is just 2.3 to 3micrometres in diameter,[20] within the size range of many bacteria.[21] At the other extreme, the shells of deep-seaxenophyophores can attain 20 cm in diameter.[22] Most of the free-living freshwater amoebae commonly found inpond water, ditches, and lakes aremicroscopic, but some species, such as the so-called "giant amoebae"Pelomyxa palustris andChaos carolinense, can be large enough to see with the naked eye.
The meiosis-specificrecombinase,Dmc1, is required for efficient meiotichomologous recombination, andDmc1 is expressed inEntamoeba histolytica.[32] The purified Dmc1 fromE. histolytica formspresynaptic filaments and catalysesATP-dependenthomologous DNA pairing and DNA strand exchange over at least several thousandbase pairs.[32] The DNA pairing and strand exchange reactions are enhanced by the eukaryotic meiosis-specific recombination accessory factor (heterodimer) Hop2-Mnd1.[32] These processes are central to meiotic recombination, suggesting thatE. histolytica undergoes meiosis.[32]
Studies ofEntamoeba invadens found that, during the conversion from thetetraploiduninucleatetrophozoite to the tetranucleate cyst,homologous recombination is enhanced.[33] Expression of genes with functions related to the major steps of meiotic recombination also increase during encystations.[33] These findings inE. invadens, combined with evidence from studies ofE. histolytica indicate the presence of meiosis in theEntamoeba.
Since the Amoebozoa diverged early from theeukaryotic family tree, these results suggest that meiosis was present early in eukaryotic evolution. Furthermore, these findings are consistent with the proposal of Lahr et al.[36] that the majority of amoeboid lineages are anciently sexual.
Amoebae have been found to harvest and grow the bacteria implicated inplague.[41] Amoebae can likewise play host to microscopic organisms that are pathogenic to people and help in spreading such microbes. Bacterial pathogens (for example,Legionella) can oppose absorption of food when devoured by amoebae.[42]The currently generally utilized and best-explored amoebae that host other organisms are Acanthamoeba castellanii and Dictyostelium discoideum.[43]Microorganisms that can overcome the defenses of one-celled organisms can shelter and multiply inside them, where they are shielded from unfriendly outside conditions by their hosts.
The first illustration of an amoeboid, from Rösel von Rosenhof'sInsecten-Belustigung (1755)
The earliest record of an amoeboid organism was produced in 1755 byAugust Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus" ("the Little Proteus").[44] Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known asAmoeba proteus.[45] The term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid.[46]
In 1822, the genusAmiba (from theGreek ἀμοιβήamoibe, meaning "change") was erected by the French naturalistBory de Saint-Vincent.[47][48] Bory's contemporary,C. G. Ehrenberg, adopted the genus in his own classification of microscopic creatures, but changed the spelling toAmoeba.[49]
In 1841,Félix Dujardin coined the term "sarcode" (from Greek σάρξsarx, "flesh", and εἶδοςeidos, "form") for the "thick, glutinous, homogeneous substance" which fills protozoan cell bodies.[50]: 26 Although the term originally referred to the protoplasm of any protozoan, it soon came to be used in a restricted sense to designate the gelatinous contents of amoeboid cells.[10] Thirty years later, the Austrian zoologistLudwig Karl Schmarda used "sarcode" as the conceptual basis for his division Sarcodea, aphylum-level group made up of "unstable, changeable" organisms with bodies largely composed of "sarcode".[51]: 156 Later workers, including the influentialtaxonomistOtto Bütschli, amended this group to create the class Sarcodina,[52]: 1 a taxon that remained in wide use throughout most of the 20th century.[53]
For convenience, all amoebae were grouped as Sarcodina and generally divided intomorphological categories, on the basis of the form and structure of theirpseudopods. Amoebae with pseudopods supported by regular arrays ofmicrotubules (such as the freshwaterHeliozoa and marineRadiolaria) were classified asActinopoda, whereas those with unsupported pseudopods were classified asRhizopoda.[54] The Rhizopods were further subdivided into lobose, filose, plasmodial and reticulose, according to the morphology of their pseudopods. During the 1980s, taxonomists reached the following classification, based exclusively on morphological comparisons:[55][53]
Rhizopodavon Siebold 1845: amorphous amoebae that lack axopodia and move through pseudopodia.[55]: 41 [53]: 202
HeteroloboseaPage & Blanton 1985: amoebae with eruptive pseudopodia, similar to the lobose ones but with a distinct movement, and usually with flagellate life stages. It was traditionally divided into those which aggregate to form fruiting bodies (Acrasida) and those that do not (Schizopyrenida).[53]: 203–204
Testacealobosiade Saedeleer 1934: lobosetestate amoebae. This polyphyletic group included three unrelated lineages of amoebozoans enclosed by tests or other complex coverings:Arcellinida,Himatismenida andTrichosida.
CaryoblasteaMargulis 1974: amoebae with sparse, non-motile flagella on the surface. This group only includes the orderPelobiontida,[53]: 207 which now belongs to the amoebozoan groupArchamoebae together with some naked amoebae.[56]
EumycetozoeaZopf 1885: plasmodial amoebae with filiform subpseudopodia that produce fruiting bodies.
PlasmodiophoreaCook 1928: endoparasitic plasmodial amoebae with minute pseudopodia. This group is now an order withinRhizaria, closely related to the endoparasitesPhagomyxida.
FiloseaLeidy 1879: amoebae with filose pseudopodia.
Aconchuliniade Saedeleer 1934: filose naked amoebae, sometimes covered in scales. This group included two unrelated taxa: thenucleariid amoebae, closely related tofungi; and most of theVampyrellida, found in Rhizaria.
Testaceafilosiade Saedeleer 1934: filose testate amoebae. This group included taxa now found throughout Rhizaria, such asGromiida andEuglyphida.
Granuloreticuloseade Saedeleer 1934: amoebae with delicate granular pseudopodia. This group included both theForaminifera (now in Rhizaria) and some members of Vampyrellida.
XenophyophoreaSchulze 1904: plasmodial amoebae enclosed in a branched-tube system composed of a transparent organic substance. This group is now fully integrated into the Foraminifera.
ActinopodaCalkins 1909: spherical amoebae that float in the water column. This group included those organisms that have aheliozoan-type appearance, with radially positioned filopodia, reticulopodia or axopodia surrounding the cell body. These were theRadiolaria,Phaeodaria,Proteomyxidea (all three now in Rhizaria),Centroplasthelida (now inHaptista), andActinophryida (now inStramenopiles).
The 'amoeboflagellate' hypothesis byThomas Cavalier-Smith, where higher eukaryotes evolved from amoeboid phyla.[57]: 244
In the final decades of the 20th century, a series of molecular phylogenetic analyses confirmed that Sarcodina was not amonophyletic group, and that amoebae evolved from flagellate ancestors.[10] The protozoologistThomas Cavalier-Smith proposed that the ancestor of most eukaryotes was anamoeboflagellate much like modernheteroloboseans, which in turn gave rise to a paraphyletic Sarcodina from which other groups (e.g., alveolates, animals, plants) evolved by a secondary loss of the amoeboid phase. In his scheme, the Sarcodina were divided into the more primitiveEosarcodina (with the phyla Reticulosa and Mycetozoa) and the more derivedNeosarcodina (with the phylaAmoebozoa for lobose amoebae and Rhizopoda for filose amoebae).[57]
Shortly after, phylogenetic analyses disproved this hypothesis, as non-amoeboidzooflagellates and amoeboflagellates were found to be completely intermingled with amoebae. With the addition of many flagellates to Rhizopoda and the removal of some amoebae, the name was rejected in favour of a new nameCercozoa. As such, both names Rhizopoda and Sarcodina were finally abandoned as formal taxa, but they remained useful as descriptive terms for amoebae.[58]: 238 The phylum Amoebozoa was conserved, as it still primarily included amoeboid organisms, and now included the Mycetozoa.[58]: 232
AmoebozoaLühe 1913 em. Cavalier-Smith 1998: includes all naked and testate lobose amoebae (traditional Lobosea) as well as thepelobionts andeumycetozoans, and a few flagellates.[60]: 17
HeteroloboseaPage & Blanton 1985: amoebae with lobose pseudopodia but eruptive flow of cytoplasm. Currently it includes the aggregative Acrasida, as well as several other amoeboflagellates. They are a class ofexcavates closely related toEuglenozoa, with whom they share their characteristic discoidalmitochondrial cristae.[10]: 293
StramenopilesPatterson 1989 em. Adl et al. 2005: although primarily composed by flagellates, it contains a few groups of amoebae. For example: theActinophryida, an order with typical heliozoan morphology;[10]: 293 the amoeboidRhizochromulina, a genus ofchrysophytes;[61] orSynchroma, a genus of amoeboid algae with reticulate axopodia.[62]
RotosphaeridaRainer 1968: also known asnucleariids, includes a few filose amoebae traditionally classified within the Filosea, positioned as the sister group ofFungi.[63]
CentroplasthelidaFebvre-Chevalier & Febvre 1984: heliozoans with a centroplast from which axopodia arise.[10]: 293 They are closely related to thehaptophyte algae inside the supergroupHaptista.[64]
RigifilidaCavalier-Smith 2012: a small order of filose amoebae previously interpreted as nucleariids.[65] Together with the flagellate ordersMantamonadida andDiphylleida, it composes theCRuMs clade, positioned closest toAmorphea.[66]
Somemulticellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboidwhite blood cells pursue invading organisms, such as bacteria and pathogenic protists, and engulf them byphagocytosis.[68]Sponges exhibit atotipotent cell type known asarchaeocytes, capable of transforming into the feeding cells orchoanocytes.[69]
Amoeboid stages also occur in the multicellular fungus-like protists, the so-calledslime moulds. Both the plasmodial slime moulds, currently classified in the classMyxogastria, and the cellular slime moulds of the groupsAcrasida andDictyosteliida, live as amoebae during their feeding stage. The amoeboid cells of the former combine to form a giantmultinucleate organism,[70] while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating "slug" which functions as a single organism.[8]
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